Technique for limiting transmission of fault current

ABSTRACT

A new type of superconducting fault current limiter is disclosed, which can advantageously be used with high voltage transmission networks. The circuit is electrically connected to two terminals, which connect to the transmission network. The superconducting circuit is located within an enclosure or tank, which is electrically isolated from ground. Therefore, the voltage difference between the enclosure and the superconducting circuit, and between the enclosure and the terminals are significantly less than exist in current deployments. In some embodiments, the enclosure is electrically connected to one of the terminals, while in other embodiments, the enclosure is electrically isolated from the terminals. The circuit can be combined with other like circuits to address a wide range of current transmission network configurations.

This application is a continuation of U.S. patent application Ser. No.13/827,991, filed Mar. 14, 2013, which is a continuation of U.S. patentapplication Ser. No. 12/818,454, filed Jun. 18, 2010, now issued as U.S.Pat. No. 8,467,158, which claims priority of U.S. Provisional PatentApplication Ser. No. 61/220,825, filed Jun. 26, 2009, the disclosures ofwhich are incorporated herein by reference in their entireties.

FIELD

The present application relates to a technique for limiting transmissionof fault current.

BACKGROUND

In electric power transmission and distribution networks, fault currentconditions may occur. A fault current condition is an abrupt surge inthe current flowing through the network caused by faults or shortcircuits in the network. Causes of the faults may include lightningstriking the network, and downing and grounding of the transmissionpower lines due to severe weather or falling trees. When faults occur, alarge load appears instantaneously. The network, in response, delivers alarge amount of current (i.e. overcurrent) to this load or, in thiscase, the faults. This surge or fault current condition is undesirableas the condition may damage the network or equipments connected to thenetwork. In particular, the network and the equipments connected theretomay burn or, in some cases, explode.

One of the systems used to protect power equipments from damages causedby fault currents is a circuit breaker. When a fault current isdetected, the circuit breaker mechanically opens the circuit anddisrupts the overcurrent from flowing.

Another system to limit the fault current is a superconducting faultcurrent limiter (“SCFCL”). Generally, a SCFCL comprises asuperconducting circuit that exhibits almost zero resistivity belowcritical temperature level T_(c), critical magnetic field level H_(c),and critical current level I_(c). If at least one of the conditions israised above the critical level, the circuit becomes quenched andexhibits resistivity.

During normal operation, superconducting circuit of SCFCL is maintainedbelow T_(c), H_(c), and I_(c). During fault, one or more the conditionsis raised above the critical level T_(c), H_(c), and I_(c).Instantaneously, the superconducting circuit in the SCFCL is quenchedand its resistance surges, thereby limiting transmission of the faultcurrent. Following some time delay and after the short circuit fault iscleared, T_(o), H_(o), and I_(o) are returned to normal values andcurrent is transmitted through the network and the SCFCL.

The superconducting fault current limiters used currently in powertransmission network, although capable of limiting fault currents, arenot adequate. For example, SCFCL are incapable of supporting highvoltage transmission networks. In addition, conventional SCFCL requireslarge footprint due to its technical deficiencies. Accordingly, a newtype of SCFCL is needed.

SUMMARY

New techniques for limiting transmission of fault current are disclosed.In one particular exemplary embodiment, the technique may be realizedwith a new type of apparatus for limiting transmission of fault current.The apparatus may comprise an enclosure electrically decoupled fromground, such that said enclosure is electrically isolated from groundpotential; first and second terminals, at least one of which iselectrically connected to one or more current carrying lines; and afirst superconducting circuit contained in said enclosure, said firstsuperconducting circuit electrically connected to said first and secondterminals.

In accordance to another embodiment of this particular exemplaryembodiment, said enclosure may comprise electrically insulatingmaterial.

Yet in accordance to another embodiment of this particular exemplaryembodiment, said enclosure may be electrically conductive.

In accordance to further embodiment of this particular exemplaryembodiment, said enclosure may be electrically isolated from said firstand second terminals.

In accordance with additional embodiment of this particular exemplaryembodiment, a first voltage is present at said first terminal and asecond voltage is present at said second terminal and where saidenclosure is at a voltage between said first and second voltages.

In accordance with yet additional embodiment of this particularexemplary embodiment, said enclosure is electrically connected to one ofsaid first and second terminals.

In accordance with still another embodiment of this particular exemplaryembodiment, the apparatus may further comprising a support forsupporting said enclosure, said support positioned between saidenclosure and ground, wherein said support electrically isolates saidenclosure from ground.

In accordance with another embodiment of this particular exemplaryembodiment, the apparatus may further comprise a coolant within saidenclosure to maintain said superconducting circuit below a predeterminedtemperature.

Yet in accordance with another embodiment of this particular exemplaryembodiment, the apparatus may further comprise a platform upon whichsaid enclosure is supported, wherein said platform is electricallyisolated from ground.

Still in accordance to another embodiment of this particular exemplaryembodiment, the apparatus may further comprise a second enclosure; thirdand fourth terminals; and a second superconducting circuit contained insaid second enclosure, said second superconducting circuit electricallyconnected to said third and fourth terminals, where said secondenclosure is supported by said platform.

Yet in accordance to another embodiment of this particular exemplaryembodiment, the apparatus may further comprise supports positionedbetween said enclosure and said platform to electrically isolate saidenclosure and said second enclosure.

In accordance to further embodiment of this particular exemplaryembodiment, said third and fourth terminals are each electricallyconnected to one or more current distribution lines.

In accordance with additional embodiment of this particular exemplaryembodiment, said third terminal may be electrically connected to one ofsaid first and second terminals.

In accordance with yet additional embodiment of this particularexemplary embodiment, the apparatus may further comprise third andfourth terminals, at least one of which is electrically connected to oneor more current carrying lines; and a second superconducting circuitcontained in said enclosure, said second superconducting circuitelectrically connected to said third and fourth terminals.

In accordance with still another embodiment of this particular exemplaryembodiment, the apparatus may further comprise fifth and sixthterminals, at least one of which is electrically connected to one ormore current carrying lines; and a third superconducting circuitcontained in said enclosure, said third superconducting circuitelectrically connected to said fifth and sixth terminals.

In accordance with another embodiment of this particular exemplaryembodiment, clearance length between the first superconducting circuitand the enclosure is less than 250 cm.

Yet in accordance with another embodiment of this particular exemplaryembodiment, the clearance length between the first superconductingcircuit and the enclosure is about 8-25 cm.

In another particular exemplary embodiment, the technique may becomprise electrically connecting first and second terminals to one ormore current transmission lines; electrically connecting asuperconducting circuit to said first and second terminals to provide apath through which fault current passes, wherein said superconductingcircuit contained in an enclosure; and electrically isolating saidenclosure from ground.

In accordance to another embodiment of this particular exemplaryembodiment, the technique may further comprise electrically isolatingsaid enclosure from said first and second terminals.

Yet in accordance to another embodiment of this particular exemplaryembodiment, the technique may further comprising utilizing a conductiveenclosure and electrically connecting said enclosure to one of saidfirst and second terminals.

In another particular exemplary embodiment, the technique may berealized by a method comprising providing a circuit breaker, the circuitbreaker comprising a housing and a plurality of bushing insulators, thehousing being exposed to atmosphere, maintained at ground potential, andspaced apart from ground by a first vertical distance; and providing afault current limiter electrically coupled to the circuit breaker, thefault current limiter comprising an enclosure, first and secondterminals, and a first superconducting circuit contained in saidenclosure and electrically coupled to said first and second terminals,where the enclosure is spaced apart from the ground by a second verticaldistance, the second vertical distance being greater than the firstvertical distance.

The present disclosure will now be described in more detail withreference to exemplary embodiments thereof as shown in the accompanyingdrawings. While the present disclosure is described below with referenceto exemplary embodiments, it should be understood that the presentdisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein, and with respect to which the present disclosure maybe of significant utility.

BRIEF DESCRIPTION OF THE FIGURES

In order to facilitate a fuller understanding of the present disclosure,reference is now made to the accompanying drawings. These figures maynot necessarily be drawn to scale. In addition, these figures should notbe construed as limiting the present disclosure, but are intended to beexemplary only.

FIG. 1A shows an exemplary superconducting fault current limiting(SCFCL) system according to one embodiment of the present disclosure.

FIG. 1B shows an exemplary fault current limiting units that may becontained in SCFCL system of the present disclosure.

FIG. 2 shows another exemplary SCFCL system according to anotherembodiment of the present disclosure.

FIG. 3A shows another exemplary SCFCL system according to anotherembodiment of the present disclosure.

FIG. 3B shows another exemplary SCFCL system using internal shuntreactors according to another embodiment of the present disclosure.

FIG. 3C shows another exemplary SCFCL system using an external shuntreactor according to another embodiment of the present disclosure.

FIGS. 4A and 4B show side view and top view of another exemplary SCFCLsystem according to another embodiment of the present disclosure.

FIGS. 5A and 5B show side view and top view of another exemplary SCFCLsystem according to another embodiment of the present disclosure.

FIG. 6 shows another exemplary SCFCL system according to anotherembodiment of the present disclosure.

FIG. 7 shows another exemplary SCFCL system according to anotherembodiment of the present disclosure.

DETAILED DESCRIPTION

Herein, several embodiments of a superconducting fault current limiterare introduced. Those of ordinary skill in art will recognize that theembodiments included in the present disclosure are for illustrativepurpose only. For purpose of clarity and simplicity, parts, features,and functions already known may be omitted.

Referring to FIGS. 1A and 1B, there are shown an exemplarysuperconducting fault current limiting (SCFCL) system 100 according toone embodiment of the present disclosure. In the present embodiment,SCFCL system 100 may comprise one or more modules 110. However, SCFCLsystem 100 of the present disclosure may contain any number of modules110. For example, SCFCL system 100 may have three, single phase modulesthat are identical to one another. The modules may be serially connectedor connected in a parallel circuit. For the purposes of clarity andsimplicity, the description of SCFCL system 100 will be limited to onesingle phase module 110.

The module 110 of SCFCL system 100 may comprise an enclosure or tank 112defining a chamber therein. In one embodiment, the enclosure or tank 112may be thermally and/or electrically insulating tank 112 such as thosemade with fiberglass or other dielectric material. In the presentembodiment, the tank 112 is a metallic tank 112 comprising an inner andouter layers 112 a and 112 b, and a thermally and/or electricallyinsulating medium interposed therebetween.

Within the tank 112, there may be one or more fault current limitingunits 120 which, for the purpose of clarity and simplicity, are shown asa block. In the present embodiment, the module 110 may be a single phasemodule 110 having a single phase fault current limiting unit 120. Inanother embodiment, the module 110 may be a three phase module havingthree, single phase fault current limiting units 120. As illustrated inFIG. 1B one or more superconducting circuits 122 and first and secondend caps 124 and 126 may be disposed in the fault current limiting units120. The first and second end caps 124 and 126, in one embodiment, maybe corona shields. In the present disclosure, the fault current limitingunit 120 may have a clearance length of 250 cm or less from the tank112. Preferably, the clearance length may be 8-25 cm. Herein, theclearance length may refer to the shortest distance between the one ormore superconducting circuits 122 and the tank 112.

Returning to FIG. 1A, the module 110 may also comprise one or moreelectrical bushings 116. The bushings 116 may comprise an innerconductive material (not shown) and an outer insulator. The distal endof the bushings 116 may be coupled to a respective current line 142 viaterminals 144 and 146 to couple SCFCL module 110 to the transmissionnetwork (not shown). The current lines 142 may be transmission linesused to transmit power from one location to another (e.g. current sourceto current end users), or power or current distribution lines. The innerconductive material in the bushings 116 may connect the terminal 144 and146 of the bushing 116 to the fault current limiting unit 120.Meanwhile, the outer insulator is used to insulate the enclosure or tank112 from the inner conductive material, thereby allowing the tank 112and the terminals 144 and 146 to be at different electrical potentials.If desired, SCFCL module 110 may comprise optional internal shuntreactor 118 or an external shunt reactor 148, or both, to connect theconductive material contained in the electrical bushings 116. However,the present disclosure does not preclude SCFCL module 110 without theinternal shunt reactor 118 or an external shunt reactor, or both.

The temperature of one or more fault current limiting units 120 may bemaintained at a desired temperature range by coolant 114 contained inthe tank 112. In one embodiment, it may be desirable to maintain thefault current limiting units 120 at a low temperature, for example, ˜77°K. To maintain at such a low temperature range, liquid nitrogen orhelium gas may be used as coolant 114. In other embodiment, it may bedesirable to maintain the temperature of the one or more fault currentlimiting units 120 at other temperature range, and other types ofcoolant, in gaseous or liquid form, may also be used. For example, itmay be desirable to maintain the temperature of the fault currentlimiting units 120 at a room temperature. In such a case, air maintainedat a room temperature may also be used as the coolant 114. Whenintroduced, the coolants 114 may enter the tank 112 via a feed line (notshown) and a port 115 coupled to the tank 112. In the presentdisclosure, the feed line and the port 115 may preferably be made fromthermally and/or electrically insulating material. However, the presentdisclosure does not preclude any one of the feed line and the port 115from containing thermally and/or electrically conductive material. Ifthe feed line and the port 115 do not provide grounding of the tank 112or any component contained therein, they may be made from any type ofmaterial.

In the present embodiment, the tank 112 may be supported from the groundby an optional external support 134. Meanwhile, the fault currentlimiting units 120 may be supported from the tank 112 by an optionalinternal support 132. Those of ordinary skill in the art may recognizethat both of the internal supports 132 may be optional as the faultcurrent limiting units 120 may be supported from the tank 112 by someother components. Even if included, the internal support 132 may supportthe fault current limiting units 120 from the side or top of the faultcurrent limiting units 120, not necessarily from the bottom. Likewise,the tank 112 may be supported from the ground by some other components.Moreover, if included, the external support 134 may support the tankfrom the side or top of the tank 112, not necessarily from the bottom ofthe tank 112.

If included, each of the internal support 132 and the external support134 may preferably be made from thermally and/or electrically insulatingmaterial. However, the present disclosure does not preclude thermallyand/or electrically conductive internal support 132 and the externalsupport 134. If thermally and/or electrically conductive externalsupport 134 is used, it may be desirable to provide an electricallyinsulating material between the support 134 and the ground toelectrically isolate the tank 112 from the ground.

SCFCL system 100 having one or more SCFCL modules 110 may beincorporated into the current distribution network. The tank 112 may beeither electrically connected to the transmission lines 142. However,the tank 112 may be electrically decoupled from the ground by air, whichmay act as an insulator, and optionally by the external support 134. Asnoted above, the tank 112 may be metallic tank 112 in some embodiments.In these embodiments, the tank 112 may typically float at a voltage thatis close to the voltage of the terminals 144 and 146. It is contemplatedthat during normal operation, the voltage of the tank 104 may beapproximately the same as the voltage being transmitted on thetransmission lines 142 as the voltage at transmission lines 142 a, 142 bremain nearly identical. During fault, the voltage of the metal tank maybe between the voltage at transmission lines 142 a and 142 b.

During normal operation, current from the transmission line 142 istransmitted through the SCFCL module 110 from the first terminal 144 tothe second terminal 146 via the bushings 116 and the fault currentlimiting unit 120. In the fault current limiting unit 120, thesuperconducting circuit 122 therein is maintained below criticaltemperature (T_(c)), critical magnetic field (H_(c)), and criticalcurrent (I_(c)) and remains in a superconducting state. The resistance,therefore, may be negligible compared to system impedance. When a faultoccurs, the fault current transmitted to the module 110 may cause atleast one of the operating parameters, temperature (T_(o)), Magneticfield (H_(o)), and current (I_(o)) of the superconducting circuit 122 toincrease above the critical limit T_(c), H_(c), or I_(c). The resistanceof the superconducting circuit 122 increases, to decrease thetransmission through SCFCL module 110 of fault current to predefinedvalue, preferably about 50%. Limited current using SCFCL may vary from10% to 90%, depending on the systems operators' specification. After thefault current subsides, the superconducting circuit 122 may return tosuperconducting state and hence to nearly zero resistance. This processis referred as the recovery cycle.

In the present disclosure, the tank 112 of the module 110 iselectrically decoupled from the ground. Moreover, high voltage designchallenges may be addressed outside of the module 110 by, for example,air which may act as an electrical insulator. As a result, high voltagebushing design and special high voltage design within the tank 112,which otherwise must be included to operate under high voltage, may beunnecessary. The design of the module 110, therefore, may be simplified.Moreover, the dimension of the module 110 may be much smaller. Even ifthe tank 112 of the module 110 is made from electrically conductivematerial, the tank 112 may be maintained at a non-zero electricalpotential by, for example, removing the tank 112 from the ground andelectrically floating the tank 112 with respect to ground. The tank 112may be at a different voltage, based on the amount of electricalisolation between the tank, the network and ground. In some embodiments,the support structures 132 and 134 form a voltage divider between themodule 110 and ground and wherein the electrical potential of the tank112 may be a value above ground and below terminal voltage.

In the present disclosure, the module 110 may preferably have thefollowing exemplary specifications.

-   -   Maximum voltage during fault at rated limited fault current=10        kV rms    -   Maximum load current=1.2 kA rms    -   Maximum fault current=40 kA rms    -   Maximum impedance during fault at rated limited fault        current=0.25Ω.

In other words, the impedance of the current limiter module 110 canincrease to 0.25Ω during fault conditions. The module 110 can withstanda maximum fault current of 40 kA rms. Based on these values, the maximumvoltage potential across the module 110 may be 0.25Ω*40 kA rms, or 10 kVrms. Knowing these parameters, it is possible to design a variety oftransmission networks, which use various system voltages and havemaximum fault current less than would otherwise be possible.

If the network requires a 3 phase system having a system voltage of 138kV, 1200 A, and 63 kA fault transmission line. Based on thesecharacteristics, the following specifications can be calculated:

-   -   Phase voltage (which is defined as

$\frac{systemvoltage}{\sqrt{3}}\text{)}$is 80 kV.

-   -   The system short circuit impedance is calculated by dividing the        phase voltage by the maximum fault current, or

$\frac{80\mspace{14mu}{kV}}{63\mspace{14mu}{kA}} = {1.26\mspace{11mu}\Omega}$

-   -   The maximum fault current allowed by the SCFCL module is 40 kA.        So, the current reduction requirement is at least

$\frac{{63\mspace{14mu}{kA}} - {40\mspace{14mu}{kA}}}{63\mspace{14mu}{kA}},$or 37%.

To achieve a maximum fault current of less than 40 kA, the impedance ofthe network may be greater than

$\frac{80\mspace{14mu}{kV}}{40\mspace{14mu}{kA}},$or 2Ω. The number of SCFCL modules can be determined by subtracting thesystem short circuit impedance (1.26Ω) from the desired impedance(2.0Ω), and dividing this result by 0.25Ω (the impedance of one SCFCLmodule). Allowing for margin, the number of 10 kV SCFCL units requiredcan be determined to be 4. Based on this, the total SCFCL impedance ofall the modules is 4*0.25, or 1.0Ω. The limited fault current is nowgiven by

$\frac{80\mspace{14mu}{kV}}{{1.26\mspace{14mu}\Omega} + {1.0\mspace{14mu}\Omega}},$or 35 kA. The fault current reduction achieved is given by

$\frac{{63\mspace{14mu}{kA}} - {35\mspace{14mu}{kA}}}{63\mspace{14mu}{kA}},$or 44%.

Table 1 shows other configurations that can be employed. In theembodiments shown below, the limited current is maintained below 40 kA.The system voltage, and therefore the phase voltage, of eachconfiguration is varied, and the resulting number of SCFL is computed.Based on this, the limited current and % current reduction can becalculated.

TABLE 1 Requirements for various system configurations ProsepectiveSystem short SCFCL SCFCL Number of System voltage - Phase fault circuitUnit unit SCFCL Limited Line-to-Line Voltage current impedance, voltageImpedance units current % Current [kV rms] [kV rms] [kA rms] Zs [Ohm][KV rms] [Ohm] required [kA rms] reduction 15 9 63 0.14 3.3 0.08 1 3938% 35 20 63 0.32 10 0.25 1 35 44% 70 40 63 0.64 10 0.25 2 35 44% 100 5863 0.92 10 0.25 3 35 45% 138 80 63 1.26 10 0.25 4 35 44% 220 127 63 2.0210 0.25 6 36 43% 345 199 63 3.16 10 0.25 10 35 44%

SCFCL system 100 comprising one or more modules 100 provides manyadvantages. As noted above, the dimension of the module 110 may be muchsmaller compare to the dimension of the module that is electricallycoupled to the ground. For example, the SCFCL module 110 of the presentdisclosure may have the clearance length of 250 cm or less. In oneembodiment, the clearance length, in one embodiment, may be as small as8-25 cm. As noted above, the clearance length may be the shortestdistance between the one or more superconducting circuits 122 and thetank 112. This may be different from the tank that may be electricallycoupled to the ground. In the latter case, the clearance length may beas much as 90 inches.

The large dimension in the latter case may be attributable to an arcpath between the superconducting circuit and the tank. During the faultcondition, temperature within the tank may increase substantially. Ifliquid nitrogen is used as the coolant, rise in temperature may causethe liquid nitrogen to form nitrogen gas bubbles. High electric fieldfrom the superconducting circuit voltage may cause the bubbles to lineup from the superconducting circuit to the tank. As nitrogen gas is apoor electrical insulator compared to liquid nitrogen, the bubbles mayprovide a weak dielectric path by which arc flashover may occur betweenthe superconducting circuit and the tank. If the tank is maintained atground potential, the arc path may be formed connecting thesuperconducting circuit, the tank, and the ground via the bubbles. As aresult, high voltage breakdown may occur causing another fault. One wayto prevent formation of such a path is to increase the clearance length,thereby decreasing the probability of formation of a complete pathbetween the tank and the superconducting circuit. However, increasingthe distance between the tank and the superconducting circuit,necessarily increase the footprint of the SCFCL module. In the presentembodiment, the tank, which is maintained above the ground voltagelevel, may prevent arcing current from flowing from the superconductingcircuit to the ground even if bubbles line up from the superconductingcircuit to the tank. As such, SCFCL 100 of the present embodiment mayprevent additional fault despite smaller footprint, and provides acompact SCFCL device with superior High Voltage performance that is incritical need for power transmission and distribution systems.

Smaller dimension also enables vertically or horizontally orientedmodules 110. Herein, vertical and horizontal orientation may refer tothe aspect ratio of the height to width of the modules 110. In oneembodiment, the modules 110 may be tall and thin modules, thusvertically oriented. In another the modules 110 may be horizontallyoriented, having short and wide dimension.

If two or more modules 110 are included in the system 100, the pluralityof smaller modules 110 may be coupled in series and/or parallel tocustomize the design for higher voltages and/or higher currentapplications. In addition, stringent testing requirements by powerutility companies that may require testing SCFCL devices at high voltageduring fault conditions can be met by the present module 110, where eachmodule 110 may be tested independently. Maintaining or performing repairwork may be done by removing and/or exchanging the used modules 110 withnew modules 110. The modules 110 of the present embodiment may be asingle phase module 100 a. In another embodiment, the module may be athree phase module. The single phase module may have advantages when atransmission line SCFCL is designed. Meanwhile, the three phase modulemay be used for lower voltage distribution system applications.

Referring to FIG. 2, there is shown another exemplary SCFCL system 200according to another embodiment of the present disclosure. It should beappreciated by those of ordinary skill in the art that SCFCL system 200of the present embodiment is similar to the system 100 of earlierembodiment. In addition, SCFCL system 200 of the present embodimentcontains many components that are similar to those included in SCFCLsystem 100 shown in FIGS. 1A and 1B. As such, figures and thedescription of SCFCL system 100 of earlier embodiment should also beread along with SCFCL system 200 of the present embodiment.

The module 210, unlike the module 110 of the SCFCL system 100, comprisesa conducting rod 216 connecting the fault current limiting unit 120 andthe tank 112 to one of the current lines 142 a and 142 b. As such, thetank 112 may be maintained at the same electrical potential as one ofthe terminals 144 and 146 during both normal and fault conditions.

If SCFCL system comprises a plurality of SCFCL modules, the modules maybe arranged in various configurations. Referring to FIG. 3A, there isshown another exemplary SCFCL system 300 a according to anotherembodiment of the present disclosure. The apparatus 300 a may be SCFCLsystem comprising a plurality of SCFCL modules 310 i-310 iii. Those ofordinary skill in the art will recognize that SCFCL system 300 a of thepresent embodiment may contain components that are similar to thosedisclosed in earlier embodiments. For purpose of clarity, description ofsimilar features will be omitted. As such, the description of SCFCLsystem 300 a of the present embodiment should be read with SCFCL systemsof earlier embodiment.

As illustrated in FIG. 3A, SCFCL system 300 a of the present embodimentmay comprise a plurality of SCFCL modules, preferably, three, singlephase SCFCL modules 310 i-310 iii. Each module 310 i-310 iii may besimilar to each other and to that disclosed in earlier embodiment. Ifdesired, one or more modules 310 i-310 iii may include internal and/orexternal shunt reactors. However, the present disclosure does notpreclude one or more modules 310 i-310 iii having no internal and/orexternal shunt reactors 318 and 348.

The tank 312 of each module 310 i-310 iii may be spaced apart andelectrically decoupled from the ground. If desired, each tank 312 may besupported by electrically and/or thermally insulating supports 334 a and334 b and a platform 336. SCFCL system 300 may also comprise coolantsupply 352 providing coolant to each tank via coolant feed line 354.

Each module 310 i-310 iii is a single phase module arranged in series.As such, current may flow through the first module 310 i into the secondmodule 310 ii and then to the third module 310 iii. Such a seriesconfiguration can be used to create the configurations shown in Table 1.Similar to the SCFCL module described above, each module 310 i-310 iiimay be horizontally or vertically oriented.

In FIG. 3B, the modules 310 i-310 iii of another exemplary SCFCL system300B may be oriented in vertical or horizontal orientation. However,each of the modules 310 i-310 iii does not contain external shutreactors. Instead, each module 310 i-310 iii may contain internal shuntreactor. In FIG. 3C, the modules 310 i-310 iii of yet another exemplarySCFCL system 300 d may be oriented in vertical or horizontalorientation. However, a single external shunt reactor 348 is used toconnect the current lines 315 a and 315 b.

Similar to the tanks 112 of SCFCL systems 100 and 200 of earlierembodiments, the tanks 312 of SCFCL system of the present embodiment maybe made from conducting material, such as metal, or dielectric material,such as fiberglass. In addition, the tanks 312 are electricallydecoupled from ground and/or from the terminals 342 a, 342 b, 344 a, 344b, 346 a, and 346 b. In the case of electrically conductive tanks 312,the voltage of each tank 312 may float at or about the voltage of thecurrent lines 3215 a and 315 b during normal operation. In the event ofa fault, each tank 312 will tend to float at a voltage between thosepresent at each of its terminals 342 a, 342 b, 344 a, 344 b, 346 a, and346 b. Since the tanks 312 may be on a common platform 336, insulatingsupports 334 a and 334 b may be used to insulate the tanks 312 from oneanother in the event of a fault, as each tank may tend to float at adifferent electrical potential.

In other embodiments, the tanks 312 are conductive and each tank 112 istied to one of its respective terminals 342 a, 342 b, 344 a, 344 b, 346a, and 346 b. In this case, insulating supports 334 b may be used toisolate the tanks 312 from the platform 336 and from one another in theevent of a fault, as each will be at a different electrical potential.In one embodiment, one tank may be electrically connected to theplatform 336, while the other tanks are electrically isolated therefrom.

Referring to FIGS. 4A and 4B, there are shown side and top views,respectively, of another exemplary SCFCL system 400 according to anotherembodiment of the present disclosure. In the present embodiment, SCFCL400 may comprise first to sixth single phase superconducting currentfault limiter modules 410 i, 410 ii, 410 iii, 410 iv, 410 v, and 410 vi.Each of SCFCL modules 410 i, 410 ii, 410 iii, 410 iv, 410 v, and 410 vimay be similar to each of SCFCL modules contained in SCFCL system 100,200, and 300 of earlier embodiments.

As illustrated in FIG. 4B, SCFCL modules 410 i, 410 ii, 410 iii, 410 iv,410 v, and 410 vi are arranged in a serial and in parallel connection.For example, the first and fourth SCFCL modules 410 i and 410 iv may beelectrically connected in a parallel connection; the second and fifthSCFCL modules 410 ii and 410 v may be electrically connected in aparallel connection; and the third and sixth SCFCL modules 410 iii and410 vi may be connected in a parallel connection. At the same time, thefirst and fourth SCFCL module 410 i and 410 iv are connected to thesecond and fifth SCFCL modules 410 ii and 410 v in series, and thesecond and fifth SCFCL modules 410 ii and 410 v are connected to thethird and sixty SCFCL modules 410 iii and 410 vi in series. Such aconfiguration can be used to allow higher currents (due to the parallelconfiguration) and higher voltages (due to series configuration). Thoseof ordinary skill in the art will realize that the disclosure is notlimited to this specific configuration, as the SCFCL modules 410 i, 410ii, 410 iii, 410 iv, 410 v, and 410 vi may be arranged in anycombination of series and parallel connections. In addition, the numberof SCFCL modules is not limited to a particular number.

Referring to FIGS. 5A and 5B, there are shown side and top views,respectively, of another exemplary SCFCL system 500 according to anotherembodiment of the present disclosure. In the present embodiment, SCFCL500 comprises a SCFCL module 510. SCFCL module 510 of the presentembodiment may comprise a tank 512 made from a thermally and/orelectrically insulating material such as, for example, fiberglass orG10. However, a conductive tank 512 such as, for example, a metallictank 512 is not precluded in the present disclosure. Even if the tank512 is made from electrically conducting material, the tank 512 may beelectrically isolated and decoupled from ground. In some embodiments,the tank 512 is isolated from all of the terminals 517 a-517 f, while inother embodiments, it may be electrically coupled to exactly one of theterminals 517.

In the tank 512, a plurality of SCFCL units 520 i, 520 ii, and 520 iiiare disposed. Each of SCFCL units 520 i, 520 ii, and 520 iii may besimilar to SCFCL units described in earlier embodiments. In addition,SCFCL units 520 i, 520 ii, and 520 iii of the present embodiment may bearranged in one or more configurations similar to those described inearlier embodiments. Each unit 520 i, 520 ii, and 520 iii may be asingle phase unit, and, preferably, the phase of each unit 520 i, 520ii, and 520 iii may different from the phase of adjacent unit 520 i, 520ii, and 520 iii. For example, the unit 520 i may have a phase differentfrom the unit 520 ii. Meanwhile, the unit 520 i may have a phase that isthe same or different from the phase of the unit 520 iii. In someembodiments, 3 modules are included within the tank 512, where each isone phase of a 3-phase power system. In this embodiment, each phase isoffset from each of the other two by 120°. In addition, SCFCL units 520i, 520 ii, and 520 iii may be arranged in a vertical orientation orhorizontal orientation.

The temperature of the superconducting fault current limiting units 520i, 520 ii, and 520 iii may be maintained at a desired temperature. Forexample, the superconducting fault current limiting units 520 i may bemaintained at a low temperature (such as ˜77 k) by a coolant 114 suchas, for example, liquid nitrogen or another coolant 114 (in gaseous orliquid form) introduced to the tank 512. When introduced, the coolant113 may enter the tank 512 via a feed line 354 and a port 515 coupled tothe tank 512. Similar to the earlier embodiment, the feed line and theport may be insulating feed line; although, a conductive feed line isnot precluded in the present disclosure.

Each of SCFCL units 520 i, 520 ii, and 520 iii is also coupled toelectrical bushings 516. The electrical bushings 516 of each unit may becoupled to one another by an internal shunt reactor (not shown).Alternatively, the bushings 516 may be coupled to one another via anexternal shunt reactor 548 as SCFCL of earlier embodiments. Yet inanother embodiment, SCFCL module 520 i may comprise both the internaland external shunt reactors, and the bushings 516 of each SCFCL units520 i, 520 ii, and 520 iii may be coupled to one another via bothinternal and external shunt reactors. In addition, the bushing 516 maybe coupled to the current line at terminals 517, thus coupling SCFCLsystem 500 to the transmission network (not shown).

Each of SCFCL units 520 i, 520 ii, and 520 iii may be supported byoptional internal supports 132. Meanwhile, SCFCL tank 512 may besupported by one or more optional external support 334 a and 334 b. Toaccommodate larger size of SCFCL system 500 of the present embodiment,an optional platform 336 may also be included. Much like SCFCL systemsof earlier embodiment, the internal and external supports 132 and 334 aand 334 b and the platform 336, if included, may be thermally and/orelectrically insulating based structures. However, thermally and/orelectrically conductive structures, much like the tank 512 and the feedline 354, are not precluded in the present disclosure.

The superconducting fault current limiter 500 of the present embodimentmay provide several advantages. For example, most of the high voltagedesign requirements are moved to an external support structures in airand air gap insulation. In addition, SCFCL 500 requires shorter internalgap between the units 520 i, 520 ii, and 520 iii in the tank 512, andshorter and lower voltage rated bushings 516. Further, providing threephase module may remove redundancy in space utilization that may bepresent in providing three single phase modules. In particular, theoverall dimension of SCFCL may become more compact and light. Such SCFCLsystem 500 may be attractive where smaller footprint is desired. Inaddition, SCFCL system 500 may be a part of a device level modulardesign. Such modules may be used to configure a larger unit byconnecting the units in parallel and/or series for higher current andhigher voltage applications.

Referring to FIG. 6, there is shown another exemplary SCFCL system 600according to another embodiment of the present disclosure. SCFCL system600 is similar to the SCFCL system shown in FIG. 5. However, the tank612 in the SCFCL system 600 of the present embodiment is much larger. Inaddition, the tank 612 is electrically conductive, and electricallycoupled to the ground. As such, the tank 612 is maintained at a groundpotential. Support 132 may optionally provide electrical isolationbetween the tank 612 and the units 520 i-520 iii

Referring to FIG. 7, there is shown another exemplary SCFCL system 700according to another embodiment of the present disclosure. In thepresent embodiment, the SCFCL system 700 is a three-phase system,comprising three, single phase modules 710. In addition, the system 700is provided near a circuit breaker 770. Although the SCFCL system 700 ofthe present embodiment comprises three, single phase modules 110, onlytwo modules 710 are shown for the purposes of clarity and simplicity.

The circuit breaker 770 adjacent to the SCFCL systems 700 may include ahousing 772 and a tank assembly 771. As illustrated in the figure, thehousing 772 may be spaced apart from the ground by a first verticaldistance y₁. The tank assembly 771, meanwhile, may comprise firstcylindrical tank 774. Although not shown, the tank assembly 771 mayinclude second and third cylindrical tanks. The tank assembly 771 mayalso include plurality of bushing insulators 776. Depending on the typeof the circuit breaker 770, the number of bushing insulators 776 mayvary. For example, a three-phase circuit breaker 770 may comprise a pairof first phase bushing insulators 776 a-i and 776 a-ii, a pair of secondphase bushing insulators 776 b-i and 776 b-ii, and a pair of third phasebushing insulators (not shown). Coupled to each bushing insulators,there may be a plurality of current transformers 778.

As noted above, the SCFCL system 700 of the present embodiment is athree-phase system, comprising three, single phase modules 710. Thefirst module 710 may be connected to one of the first phase bushinginsulator pair 776 a-i and 776 a-ii; the second module 710 may becoupled to one of the second phase bushing insulators 776 b-i and 776b-ii; and the third module (not shown) may be coupled to one of thethird phase bushing insulators (not shown).

Each of the SCFCL modules 710 of the present embodiment may be similarto the SCFCL module 110 of SCFCL system shown in FIG. 1. For example,each module 710 of the present embodiment may comprise a tank 712 thatmay be spaced apart from the ground and maintained at non-groundpotential. As illustrated in the figure, the tank 712 may be spacedapart from the ground by a second vertical distance y₂. In addition, thetank 712 are spaced apart from the circuit breaker 770 along thehorizontal direction 780 by a lateral distance x.

Within the tank 712, there may be one or more fault current limitingunits 720. Further, each module 710 may comprise one or more bushings716, and one or more of optional internal or external shunt reactors 718and 748. As each module 710 of the present embodiment may be similar tothe module 110 described in FIG. 1, additional description will beomitted.

The SCFCL system 700 of the present embodiment may be positionedproximate to the circuit breaker 770. Preferably, the tank 712 of eachmodule 710 may be positioned above the housing 772 or other parts of thecircuit breaker 770 that are maintained at ground potential. Forexample, the tanks 712 may be either aligned with the bushing insulators776 a-i, 776 a-ii, 776 b-i, 776 b-ii, etc. . . . , along the horizontaldirection 780, or positioned above them. As such, the second verticaldistance y₂ may be greater than the first vertical distance y₁. Bypositioning the tank 712 near or above the bushing insulators 776 a-i,776 a-ii, 776 b-i, 776 b-ii, etc. . . . , the space above the circuitbreaker 770 may be utilized.

In addition, the lateral spacing x between the tank 712 and the circuitbreaker 770 may be minimized. By aligning the tank 712 with the bushinginsulators 776 a-i, 776 a-ii, 776 b-i, 776 b-ii, etc. . . . , orpositioning above them, space between the tank 712 and the housing 772may be extended even if the lateral spacing x is decreased. For example,the spacing between the tank 712 and the housing 772 may be extendedalong the vertical direction. In the process, adequate electricalisolation may be provided to the tank 712 and the housing 772 by theair. The SCFCL system 700 and the circuit breaker 770, therefore, may bemore closely packed along the lateral direction 780, and the footprintrequirement may be minimized without compromising the electricalisolation.

In the SCFCL system having the tank maintained at the ground potential,the tank normally contacts the ground. In such a system, the lateraldistance x between the circuit breaker 770 and the tank must be greaterto provide adequate electrical isolation. The system and the circuitbreaker 770, therefore, requires greater footprint.

Several embodiments of an apparatus for limiting fault current aredisclosed. Those of the art will recognize that the present disclosureis not to be limited in scope by the specific embodiments describedherein. Indeed, other various embodiments of and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other embodiments andmodifications are intended to fall within the scope of the presentdisclosure. Further, although the present disclosure has been describedherein in the context of a particular implementation in a particularenvironment for a particular purpose, those of ordinary skill in the artwill recognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. An apparatus for limiting transmission of faultcurrent, the apparatus comprising: an electrically conductive enclosureelectrically decoupled from ground, such that said electricallyconductive enclosure is electrically isolated from ground potential;first and second terminals, at least one of which is electricallyconnected to one or more current carrying lines; and a firstsuperconducting circuit contained in said electrically conductiveenclosure, said first superconducting circuit electrically connected tosaid first and second terminals, wherein said electrically conductiveenclosure is electrically connected to one of said first and secondterminals, so that said electrically conductive enclosure is maintainedat a same electrical potential as said one of said first and secondterminals during normal and fault conditions.
 2. The apparatus of claim1, further comprising a support, for supporting said electricallyconductive enclosure, positioned between said electrically conductiveenclosure and ground, wherein said support electrically isolates saidelectrically conductive enclosure from ground.
 3. The apparatus of claim1, further comprising a coolant within said electrically conductiveenclosure to maintain said first superconducting circuit below apredetermined temperature.
 4. The apparatus of claim 1, comprising aplatform upon which said electrically conductive enclosure is supported,wherein said platform is electrically isolated from ground.
 5. Theapparatus of claim 4, further comprising: a second electricallyconductive enclosure, electrically decoupled from ground; third andfourth terminals; and a second superconducting circuit contained in saidsecond electrically conductive enclosure, said second superconductingcircuit electrically connected to said third and fourth terminals,wherein said second electrically conductive enclosure is supported bysaid platform.
 6. The apparatus of claim 5, further comprising supportsbetween said second electrically conductive enclosure and said platform,such that said electrically conductive enclosure and said secondelectrically conductive enclosure are electrically isolated.
 7. Theapparatus of claim 5, wherein said third and fourth terminals are eachelectrically connected to one or more current distribution lines.
 8. Theapparatus of claim 5, wherein said third terminal is electricallyconnected to one of said first and second terminals.
 9. The apparatus ofclaim 1, further comprising: third and fourth terminals, at least one ofwhich is electrically connected to one or more current carrying lines;and a second superconducting circuit contained in said electricallyconductive enclosure, said second superconducting circuit electricallyconnected to said third and fourth terminals.
 10. The apparatus of claim9, further comprising: fifth and sixth terminals, at least one of whichis electrically connected to one or more current carrying lines; and athird superconducting circuit contained in said electrically conductiveenclosure, said third superconducting circuit electrically connected tosaid fifth and sixth terminals.
 11. The apparatus of claim 1, whereinclearance length between said first superconducting circuit and saidelectrically conductive enclosure is less than 250 cm.
 12. The apparatusof claim 11, wherein the clearance length between said firstsuperconducting circuit and said electrically conductive enclosure isabout 8-25 cm.
 13. A method of limiting transmission of fault currentcomprising: electrically connecting first and second terminals to one ormore current transmission lines; electrically connecting asuperconducting circuit to said first and second terminals to provide apath through which fault current passes, wherein said superconductingcircuit is contained in an electrically conductive enclosure;electrically isolating said electrically conductive enclosure fromground; and electrically connecting said electrically conductiveenclosure to one of said first and second terminals, so that saidelectrically conductive enclosure is maintained at a same electricalpotential as said one of said first and second terminals during normaland fault conditions.
 14. A method of limiting fault current, the methodcomprising: providing a circuit breaker, the circuit breaker comprisinga housing and a plurality of bushing insulators, the housing beingexposed to atmosphere, maintained at ground potential, and spaced apartfrom ground by a first vertical distance; and providing a fault currentlimiter electrically coupled to the circuit breaker, the fault currentlimiter comprising an electrically conductive enclosure, first andsecond terminals, and a first superconducting circuit contained in saidelectrically conductive enclosure and electrically coupled to said firstand second terminals, wherein said electrically conductive enclosure iselectrically connected to one of said first and second terminals andspaced apart from the ground by a second vertical distance, the secondvertical distance being greater than the first vertical distance.